1. Field of the Invention
The present invention relates to an angular velocity sensor that is designed to detect the shaking of a video camera, the motion of an object in a virtual-reality apparatus, or directions in a car-navigation system.
This application claims priority of Japanese Patent Application No. 2004-024372, filed on Jan. 30, 2004, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
Angular velocity sensors widely used in public sector are so-called gyro-type angular velocity sensors. The vibration gyro-type angular velocity sensor has a rod-shaped vibrator and a piezoelectric element. The vibrator is vibrated at a predetermined resonant frequency. The piezoelectric element detects the Coriolis force generated due to the angular velocity. The sensor determines the angular velocity from the Coriolis force.
The vibrator may be shaped like either a rectangular prism or a tuning fork. The vibrator shaped like a turning fork is supported at two vibration nodes by electrically conductive members.
FIGS. 1A, 1B and 1C illustrate a method of supporting a conventional angular velocity sensor. As shown in FIGS. 1A, 1B and 1C, the vibrator 96 of this sensor, which is a piezoelectric member, has a groove 95. The groove 95 is cut in one face of the vibrator 96 and extends in the lengthwise direction of the vibrator 96, along almost the centerline of that face. Thus, the groove 95 divides the face into two parts 91a and 91b. Connectors 40a and 41a mechanically and electrically connect conductive support members 40 and 41 to the vibrator 96. More precisely, solder or electrically conductive adhesive is applied to connect the support members 40 and 41 to the vibrator 96.
In the conventional angular velocity sensor, a drive signal is applied to the junction between the support members 40 and 41, vibrating the vibrator 96. When the vibrator 96 rotates around its longitudinal axis, a Coriolis force develops. Two signals are generated at the faces 91a and 91b of the vibrator 96. These signals are proportional to the Coriolis force and have opposite polarities. The signals are extracted from the support member 40. The angular velocity signal can be detected from the signals.
The support members 40 and 41 must have two contradicting functions. One function is to constrain the vibration nodes. The other function is to allow the vibration nodes to move to some extent. The more the vibration nodes are constrained, the smaller the vibration. Consequently, the angular velocity sensor exhibits but a low sensitivity to angular velocity. Conversely, the less the vibration nodes are constrained, the more the vibration nodes move. In this case, the angular velocity sensor has unstable operating characteristics.
In recent years, devices have grown smaller and smaller. The support members 40 and 41 therefore become shorter as long as the members 40 and 41 extend straight as shown in FIG. 1A to 1C. As a result, they support the vibrator 96 more rigidly. To support the vibrator 96 loosely, thereby to allow the vibrator 96 to vibrate freely, the support members 40 and 41 may be bent as the support members 50 and 51 shown in FIG. 2. (See, for example, Japanese Patent Application Laid-Open Publication No. H10-332379.)
Insert moldings are used to fix the support members in place, in order to enhance the productivity of angular velocity sensors and to reduce the manufacturing cost thereof. To form the insert moldings with ease, however, the upper support member 50 and the lower support member 51 must be moved to take positions that are symmetrical with respect to a vibration node.
The method that is generally employed to electrically connect the electrodes of an angular velocity sensor is soldering. The solder used is one selected as desirable in view of the use and material of the objects to be connected together. To cope with the environmental problem, any solder made of mainly metal lead is no longer used. Instead, tin-silver-copper alloys and tin-zinc alloys are now mainly used as lead-free solders. These lead-free solders are disadvantageous. They have higher melting points than the existing tin-lead solder, and their melting points cannot be lowered even if their composition is changed or additives are used in them.
The manufacturing cost of sensors should be lowered to win the recent competition in terms of price. To this end, the functional components of sensors are assembled by means of surface mounting such as reflow. Some measures should be taken not to degrade the performance and quality of any component after the component is subjected to the surface mounting. That is, the components of any sensor must withstand not only the temperature changes in normal use condition, but also the high-temperature profile while they are being mounted. This makes it difficult to design sensors.
The lead-free solders that are now used in the surface mounting to help accomplish the environmental protection have melting points higher than the tin-lead solder. Inevitably, the reflow temperature increases. As a consequence, the sensor components are exposed to a high-temperature profile. Since the lead-free solder has a high melting point, it must be used to achieve electrical connection within each component. The heat applied during the reflow may melt or loosen the junctions between the parts of the component. In some cases, an under-fill agent or the like is used, preventing the parts from moving even if the heat melts the solder. However, a component that cannot be rigidly supported, such as the vibrator, cannot be connected by the use of a lead-free solder. Such a component must be connected to the support member with an electrically conductive adhesive that would not melt after it is cured.
The electrically conductive adhesive comprises a thermosetting resin (e.g., epoxy resin) and metal filler (i.e., electrically conductive material) dispersed in the resin at high ratio. Unless the adhesive shrinks after cured, it cannot acquire electrical conductivity. In theory, the resin would not change in properties once cured, and those parts which have become electrically conductive to one another due to dielectric breakdown in the metal filler would not undergo any changes such as resistance change. In practice, however, those parts of the resin, which have undergone dielectric breakdown, are restored to the initial state because of the heat and moisture applied to them. The resistance of the junction between the components greatly changes, or the components become electrically insulated from one another. Further, the components connected with the electrically conductive adhesive have their initial resistance much changed, depending upon their materials. The product, in which the components are thus connected, may be impaired in terms of reliability.
The base of the vibrator may be made of amorphous carbon, which has good temperature characteristic. In this case, it is difficult to connect the vibrator to support members by soldering since amorphous carbon is dense and chemically stable. It is more difficult to form film on the vibrator by means of, for example, plating. Even if metal film is successfully formed on the vibrator, to provide electrodes, it will not adhere firmly enough to impart mechanical connection strength to the vibrator. No method is available, which can reliably connect the vibrator to the support members, both electrically and mechanically. This renders it hard to provide reliable angular velocity sensors.